The present invention relates to a gasket, and more specifically, relates to the gasket which can improve a seal performance without causing an insufficient fastening force even in a case wherein a fastening force by a fastening device is low.
There is proposed a gasket wherein a hardness lowering area where the hardness is locally reduced is provided only at one portion of a seal bead annularly formed in a ferritic stainless steel plate (for example, see Japanese Patent Application Publication No. 2000-027999).
The gasket retains high hardness portions except for the hardness lowering area to obtain a spring characteristic, and an elongation in the hardness lowering area of the seal bead so as to prevent an edge portion of the seal bead from cracking. Incidentally, a metal plate forming the gasket is an austenitic stainless steel plate represented by SUS301-CSPH and SUS304-CSPH or the ferritic stainless steel plate represented by SUS430 specified in JIS (Japanese Industrial Standards), and the hardness thereof is set at a Vickers hardness value of 400HV to 550HV. Also, the hardness of the hardness lowering area is set at the Vickers hardness value of 170HV to 445HV.
Recently, in order to improve fuel consumption of an engine, thicknesses of a cylinder head and a cylinder block are reduced to attain weight reduction and low rigidity. Consequently, with the reduction of the thicknesses of the cylinder head and the cylinder block, a bolt fastening force of the gasket is reduced.
Thus, in a case wherein the bolt fastening force is reduced, in the gasket described in Japanese Patent Application Publication No. 2000-027999, even a portion where the reinforcement of the seal performance is unnecessary becomes the high hardness as well, so that when the cylinder head and the cylinder block are fastened, the gasket causes the insufficient fastening force so as to deteriorate the seal performance.
The present invention is made in view of the aforementioned problem, and an object of the present invention is to provide a gasket and a method for manufacturing a gasket which can improve the seal performance without causing the insufficient fastening force even in the case wherein the fastening force by the fastening devices is low.
Further objects and advantages of the invention will be apparent from the following description of the invention.